Electronic balances are in many cases calibrated by means of an internal calibration weight. To perform a calibration, a calibration weight of a defined mass is brought into force-transmitting contact with a force-transmitting device that is arranged in a force-measuring cell of a balance or with the load-receiving part of the force-measuring cell, whereupon a reference value is determined. Based on this reference value, further weighing parameters of the balance can be updated. After the calibration has been successfully completed, the contact between the calibration weight and the force-transmitting device or the load-receiving part is released again and the calibration weight is secured in a rest position. In the foregoing process, the calibration weight is moved by a transfer mechanism from a rest position to a calibrating position and subsequently returned to the rest position. In the calibrating position, the calibration weight is in force-transmitting contact with the force-measuring cell or with the load-receiving part; in the rest position there is no force-transmitting contact. The calibration weight arrangement and the force-measuring cell in many balances are arranged one behind the other, in the manner disclosed in the applicant's U.S. Pat. No. 6,194,672 B1 to Burkhard, et al.
There are a variety of transfer mechanisms for moving a calibration weight, wherein the latter in its rest position is in most cases seated on a support element that is connected to a lifting system.
A calibration weight arrangement with a calibration weight, as disclosed in the applicant's U.S. Pat. No. 5,148,881 to Leisinger, is moved in the vertical direction by means of pairs of wedges which can be slid horizontally towards each other, whereby the calibration weight is brought into force-transmitting contact with the force-measuring cell of the balance. This transfer mechanism is powered by a motorized drive-mechanism through a spindle that is connected to the wedges.
A device described in the Burkhard '672 patent likewise effects a vertical lifting and lowering of a calibration weight. The weight rests on a support element that is moved by a transfer mechanism with a cam-disk lifting system or an eccentric.
A calibration weight arrangement with two calibration weights that are independent of each other is disclosed in published European patent application EP 0 020 030 A1, wherein the calibration weights are coupled to and uncoupled from a calibration weight support element which is the same for both calibration weights and is connected to the load-receiving part of the force-measuring cell. The stepwise calibration has the purpose to check the calibration weight itself. As a further possibility, at least one of the calibration weights can also serve to expand the weighing range, as disclosed in U.S. Pat. No. 4,566,548 to Soedler, et al.
The aforementioned lifting elements are powered in general by small servomotors. The disadvantage in using servomotors is that a servomotor uses a relatively large amount of space in the force-measuring cell, whereby the size of the force-measuring cell as well as of the balance itself is unnecessarily increased. To improve the calibration weight arrangement, one therefore needs in particular to optimize and miniaturize the drive source of the transfer mechanism.
Especially in electronic balances of high sensitivity, the weighing result is also influenced and even altered by electrostatic charges and interactions. The servomotors used for powering the transfer mechanisms contain electrically nonconductive gearbox parts which during operation generate electrostatic fields due to friction. The electrostatic fields which occur as a result are sufficiently large to have an influence on the weighing result, particularly in balances of high sensitivity.
In Soedler '548, lifter magnets, which are energized by an electric current during the entire calibration and measurement process, are used as a drive source. Lifter magnets thus have the same disadvantage as most of the small and cost-effective drive sources, in that they have no self-locking properties. Drive mechanisms of this type are not suitable for the operation of a calibration weight arrangement for precise analytical balances, because the magnetic fields produced by them as well as the heat generated as a result of the long time periods when the coil is under current have a strong influence on the reference value and on the weighing result. The term “self-locking” in reference to a drive mechanism relates to the ability of the drive mechanism to resist the forces that act on the drive mechanism while the latter is standing still, so that the drive mechanism will not move in response to these forces.
It is therefore the object of the present invention to provide a calibration weight arrangement that is small, compact, and flexibly adaptable to different applications, and has a drive source which exerts no influence or only the smallest possible influence on the reference value to be determined and on the weighing result.